In a lithographic exposure apparatus used in the manufacture of semiconductor elements or the like, a circuit pattern formed on a mask (reticle) is transferred onto a photosensitive substrate (a wafer, for example) using a projection-optical system. A resist is coated on the photosensitive substrate, the resist is exposed using the projection-optical system, and a resist pattern corresponding to the mask pattern is produced. In the exposure apparatus, it is necessary to control the exposure dose on the photosensitive substrate. However, because the exposure dose on the photosensitive substrate cannot be measured in real time, a portion of the illumination light radiated on the mask is measured in real time, and exposure-dose control is performed based on these measurement results. Because the beam passed through the mask varies depending on the pattern ratio of the mask, it is difficult to control exposure dose by measuring the quantity of light passing through the mask. Consequently, an illumination sensor is placed so as to measure a portion of the illumination light discharged from the light source and impinging on the mask.
For example, in an exposure apparatus that uses EUV (Extreme Ultraviolet) light as the exposure light, the technology for controlling exposure dose is disclosed in Japan Patent Document No. Hei 09-184900-JP.